Thermoplastic fluxing underfill method

ABSTRACT

A flip chip having solder bumps and an underfill that is thermoplastic and fluxing, as well as methods for making such a device. The resulting device is well suited for a simple one-step application to a printed circuit board, thereby simplifying flip chip manufacturing processes.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional of U.S. patent application Ser. No.10/458,925, filed on Jun. 11, 2003, issued on Jan. 23, 2007 as U.S. Pat.No. 7,166,491. Ser. No. 11/624,916, filed Jan. 19, 2007 is also adivisional of Ser. No. 10/458,925.

BACKGROUND OF THE INVENTION

The present invention relates to a flip chip design. More particularly,the present invention relates to a flip chip design which incorporatessolder bumps and a polymeric underfill material that is thermoplasticand provides fluxing during a solder reflow operation

Electrical components such as resisters, capacitors, inductors,transistors, integrated circuits, and chip carriers are typicallymounted on circuit boards according to one of two configurations. In thefirst configuration, the components are mounted on one side of the boardand leads from the components extend through holes in the board and aresoldered on the opposite side of the board. In the second configuration,the components are soldered to the same side of the board upon whichthey are mounted. These latter devices are said to be “surface-mounted.”

Surface mounting of electronic components is a desirable technique inthat it may be used to fabricate very small circuit structures and inthat it lends itself well to process automation. A type ofsurface-mounted device, referred to as a flip chip, Chip Scale Package,or Ball Grid Array comprises an integrated circuit having numerousconnecting leads attached to pads mounted on the underside of thedevice. These surface-mounted devices are often referred to as AreaArray Packages. In connection with the use of flip chips, either thecircuit board or the device is provided with small bumps or balls ofsolder (hereinafter “bumps” or “solder bumps”) positioned in locationswhich correspond to the pads on the underside of each device and on thesurface of the circuit board. The device is mounted by (a) placing it incontact with the board such that the solder bumps become sandwichedbetween the pads on the board and the corresponding pads on the device;(b) heating the assembly to a point at which the solder is caused toreflow (i.e., melt); and (c) cooling the assembly. Upon cooling, thesolder hardens, thereby mounting the area array device to the board'ssurface. Tolerances in area array technology are critical, as thespacing between individual devices as well as the spacing between thechip and the board is typically very small. For example, spacing of flipchips from the surface of the board to the bottom of the die istypically between about 15 and about 75 mm and is expected to approachabout 10 mm in the near future.

One problem associated with area array technology is that the chips, thesolder, and the material forming the circuit board often havesignificantly different coefficients of thermal expansion. As a resultof the differing expansions, the heating of the assembly during use cancause severe stresses. The stresses imposed on the solder interconnectscan lead to failures that degrade device performance or incapacitate thedevice entirely.

In order to minimize thermomechanical fatigue resulting from differentthermal expansions, thermoset epoxies have been used. Specifically,these epoxies are used as an underfill material which surrounds theperiphery of the area array device and occupies the space beneath thechip between the underside of the chip and the board which is notoccupied by solder. Such epoxy systems provide a level of protection byforming a physical barrier which resists or reduces different expansionsamong the components of the device.

Improved underfill materials have been developed in which the epoxythermoset material is provided with a silica powder filler. By varyingthe amount of filler material, it is possible to cause the coefficientof thermal expansion of the filled epoxy thermoset to more closely matchthat of the integrated circuit and printed circuit board substrates. Inso doing, relative movement between the underside of the flip chip andthe solder connections, resulting from their differing coefficients ofthermal expansion, is minimized. Such filled epoxy thermosets thereforereduce the likelihood of device failure resulting from thermomechanicalfatigue during operation of the device.

While underfill has solved the thermal mismatch problem for area arraydevices on printed circuit boards, it has created significantdifficulties in the manufacturing process. For example, the underfillmust be applied off-line using special equipment. Typically, theunderfill is applied to up to three edges of the assembled flip chip andallowed to flow all the way under the chip. Once the material has flowedto opposite edges and all air has been displaced from under the chip,additional underfill is dispensed to the outer edges so as to form afillet making all four edges symmetrical. This improves reliability andappearance. Next, the assembly is baked in an oven to harden theunderfill. This process, which may take up to several hours, isnecessary to harden and fully cure the underfill. Thus, although theunderfill couples the area array device to the substrate replacing shearstresses with bending stresses, and provides a commercially viablesolution, a simpler manufacturing method is desirable.

Recently, attempts have been made to improve and streamline theunderfill process. One method that has shown some commercial potentialinvolves dispensing underfill before assembling the area array device tothe substrate and making solder connections. This method requires thatthe underfill allow solder joint formation to occur. Soldering of flipchips to printed circuit boards is generally accomplished by applyingflux to the solder bumps on the flip chip or to the circuit pads on theprinted circuit board. Thus, the flux must be applied to the bumpsbefore the underfill or the underfill must contain flux or have inherentproperties that facilitate solder joint formation. Flux activity isneeded to remove the oxidation on the pads for the solder to wet the padmetalization forming acceptable interconnects.

Certain underfills commonly called “dispense first underfills” or noflow underfills have been designed with self-contained flux chemistry.Unfortunately, the properties required for a good flux and thoserequired for a good underfill are not totally compatible. As such, acompromise of properties results. The best flux/underfill materialstypically require more than an hour to harden. Additionally,flux-containing underfills still require the use of special equipmentincluding automated dispensing machines.

Also, since solder assembly and underfill application are combined intoa single step, the flip chip cannot be tested until the assembly iscomplete. Thus, if the chip does not operate satisfactorily, it cannotbe removed because the underfill will have hardened, thereby preventingreworking.

In view of the above, a need still exists for a more efficient processwhich reduces the need for expensive equipment and that is compatiblewith existing electronic device assembly lines. A need for a reworkableunderfill also exists. A further need exists for a flux/underfillmaterial that can harden quickly while offering both excellent fluxingproperties and excellent underfill properties.

SUMMARY OF THE INVENTION

Briefly, therefore, the present invention is directed to a method forforming an integrated circuit assembly for attachment to a circuit boardby soldering. The method comprises applying an underfill solutioncomprising a thermoplastic resin having a glass transition temperaturethat is within the range of about −25° C. to about 60° C., a solvent,and a fluxing agent to an integrated circuit device having at least onesolder bump on a surface thereof such that the underfill solution is incontact with the at least one solder bump and with the surface of theintegrated circuit device. Then, at least a portion of the solvent isremoved from the applied underfill solution to thereby yield theintegrated circuit assembly for attachment to a circuit board, whereinthe integrated circuit assembly comprises the integrated circuit device,the at least one solder bump, and a thermoplastic fluxing underfill incontact with the integrated circuit device surface and in contact withthe at least one solder bump.

The present invention is also directed to a method for forming anintegrated circuit assembly for attachment to a circuit board bysoldering. The method comprises applying an underfill solutioncomprising a thermoplastic resin having a glass transition temperaturethat is within the range of about −25° C. to about 60° C., a solvent,and a fluxing agent to an integrated circuit device having at least onesolder bump on a surface thereof such that the underfill solution is incontact with the at least one solder bump and with the surface of theintegrated circuit device. Then, the solvent is removed from the appliedunderfill solution to thereby yield the integrated circuit assembly forattachment to a circuit board, wherein the integrated circuit assemblycomprises the integrated circuit device, the at least one solder bump,and a thermoplastic fluxing underfill in contact with the integratedcircuit device surface and in contact with the at least one solder bump.

Additionally, the present invention is directed to a method for formingan integrated circuit assembly for attachment to a circuit board bysoldering comprising applying an a underfill solution comprising athermoplastic resin, a solvent, and a fluxing agent to an integratedcircuit device having at least one solder bump on a surface thereof suchthat the underfill solution is in contact with the at least one solderbump and with the surface of the integrated circuit device. Then atleast a portion of the solvent is removed from the applied underfillsolution to thereby yield the integrated circuit assembly for attachmentto a circuit board, wherein the integrated circuit assembly comprisesthe integrated circuit device, the at least one solder bump, and a curedthermoplastic fluxing underfill in contact with the integrated circuitdevice surface and in contact with the at least one solder bump.

Further, the present invention is directed to a method for forming anintegrated circuit assembly for attachment to a circuit board bysoldering comprising applying a cured underfill film comprising athermoplastic resin and a fluxing agent to an integrated circuit devicehaving at least one solder bump on a surface thereof such that the curedunderfill film is in contact with the at least one solder bump and withthe surface of the integrated circuit device. The method also comprisesadhering the cured underfill film to the integrated circuit device toyield the integrated circuit assembly for attachment to a circuit board,wherein the integrated circuit assembly comprises the integrated circuitdevice, the at least one solder bump, and the cured thermoplasticfluxing underfill adhered to the integrated circuit device surface andthe at least one solder bump.

The present invention is also directed to a method for attaching anintegrated circuit device to a circuit board by soldering. The methodcomprises applying an underfill solution comprising a thermoplasticresin having a glass transition temperature that is within the range ofabout −25° C. to about 60° C., a solvent, and a fluxing agent to anintegrated circuit device having at least one solder bump on a surfacethereof such that the underfill solution is in contact with the at leastone solder bump and with the surface of the integrated circuit device.The applied underfill solution is dried to yield the integrated circuitassembly for attachment to a circuit board, wherein the integratedcircuit assembly comprises the integrated circuit device, the at leastone solder bump, and a thermoplastic fluxing underfill in contact withthe integrated circuit device surface and in contact with the at leastone solder bump. The integrated circuit assembly is placed onto thecircuit board to yield a circuit board with the integrated circuitassembly placed thereon. The circuit board with the integrated circuitassembly placed thereon is heated to a reflow temperature to therebysolder the integrated circuit device to the circuit board while thefluxing agent fluxes the solder and to flow the thermoplastic fluxingunderfill thereby yielding a circuit board having the integrated circuitdevice attached thereto with a metallic solder connection and thethermoplastic underfill between and bonded to the circuit board and theintegrated circuit device.

Additionally, the present invention is directed to a method forattaching an integrated circuit device to a circuit board by soldering.The method comprises applying an underfill solution comprising athermoplastic resin, a solvent, and a fluxing agent to an integratedcircuit device having at least one solder bump on a surface thereof suchthat the underfill solution is in contact with the at least one solderbump and with the surface of the integrated circuit device. The appliedunderfill solution is dried to yield the integrated circuit assembly forattachment to a circuit board, wherein the integrated circuit assemblycomprises the integrated circuit device, the at least one solder bump,and a cured thermoplastic fluxing underfill in contact with theintegrated circuit device surface and in contact with the at least onesolder bump. The integrated circuit assembly is placed onto the circuitboard to yield a circuit board with the integrated circuit assemblyplaced thereon. The circuit board with the integrated circuit assemblyplaced thereon is heated to a reflow temperature to thereby solder theintegrated circuit device to the circuit board while the fluxing agentfluxes the solder and to flow the cured thermoplastic fluxing underfillthereby yielding a circuit board having the integrated circuit deviceattached thereto with a metallic solder connection and the curedthermoplastic underfill between and bonded to the circuit board and theintegrated circuit device.

Further, the present invention is directed to a method for attaching anintegrated circuit device to a circuit board by soldering comprisingplacing an integrated circuit assembly comprising an integrated circuitdevice having at least one solder bump on a surface and a curedthermoplastic fluxing underfill in contact with said surface and incontact with the at least one solder bump onto the circuit board toyield a circuit board with the integrated circuit assembly placedthereon. The method also comprises heating the circuit board with theintegrated circuit assembly placed thereon to a reflow temperature tothereby solder the integrated circuit device to the circuit board whilethe cured thermoplastic fluxing underfill flows and fluxes the solderthereby yielding a circuit board having the integrated circuit deviceattached thereto with a metallic solder connection and the curedthermoplastic underfill between and bonded to the circuit board and theintegrated circuit device.

The present invention is also directed to a thermoplastic fluxingunderfill solution for application between an integrated circuit deviceand a circuit board to assist in solder assembly of the integratedcircuit device to the circuit board and to provide shock resistanceafter said solder assembly of the integrated circuit device to thecircuit board. The thermoplastic fluxing underfill solution comprises athermoplastic resin having a glass transition temperature within therange of about −25° C. to about 60° C., and thermal stability such thatthe thermoplastic resin loses less than about 10% of its weight uponexposure to soldering conditions comprising a temperature of about 250°C. for about 90 seconds. The solution also comprises a solvent whichdissolves the thermoplastic resin having said thermal stability and afluxing agent for fluxing a solder in solder assembly of the integratedcircuit device to the circuit board.

The foregoing and other features and advantages of the present inventionwill become more apparent from the following description andaccompanying figures.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic representation of a portion of a semiconductorwafer having solder bumps applied to its surface.

FIG. 2 is a schematic representation of a portion of a semiconductorwafer having solder bumps applied to its surface and a flux/underfillmaterial applied over the solder bumps.

DETAILED DESCRIPTION OF THE INVENTION

The present invention is directed to a thermoplastic fluxing underfillfor use during and after solder reflow operations. Advantageously, thethermoplastic fluxing underfill of the present invention is a “no-flow”type underfill and may be pre-applied to a chip and/or substrate severalmonths (e.g., at least six months) prior to the solder reflow operationwithout any decrease in the flow, adhesion, and/or reworkability.Additionally, the thermoplastic fluxing underfill of the presentinvention may be used with lead-containing and lead-free solders. Due tothese and other characteristics, the present invention is also directedto a unique method of forming an integrated circuit assembly comprisingan integrated circuit device (e.g., a flip chip) and the thermoplasticfluxing underfill that is ready for end user application (e.g., solderreflow connection onto a printed circuit board).

In general the present invention is directed to joining any appropriateelectrical component to any appropriate printed circuit board. Inaccordance with the present invention, any appropriate type electricalcomponent may, for example, comprise one or more of the following: anintegrated circuit device (e.g., a flip chip), a resistor, a capacitor,an inductor, a transistor, or an area array device). It is to be notedthat hereinafter this disclosure will be directed primarily to thejoining of an integrated circuit device to a printed circuit board.This, however, is not to be interpreted as limiting the scope of theinvention.

Appropriate substrate materials for a printed circuit board and/or anintegrated circuit device include, for example, high-pressure laminates(i.e., layers of fibrous materials bonded together under heat andpressure with a thermosetting resin). In general, a laminate layercomprises an electrical-grade paper bonded with phenolic or epoxy resinor a continuous-filament glass cloth bonded with an epoxy-resin system.Specific examples of laminate layers are: XXXPC which is an electricalpaper impregnated with phenolic resin; FR-2 which is similar to XXXPCwith a flame retardant property; FR-3 which is a self-extinguishinglaminate of electrical paper and epoxy resin; G-10 which is a laminateof glass cloth sheets and epoxy resin; FR-4 which is aself-extinguishing laminate similar to G-10; G-11 which is a glass clothand epoxy mixture; FR-5 which is a flame-resistant version of G-11. Inone embodiment of the present invention, the organic circuit boardmaterial is an FR-4 laminate layer that is placed on top of, and inintimate contact with the passive component pattern, and the two arelaminated together. In addition to laminated organic materials, thesubstrate to which the integrated circuit is bonded may comprise, forexample, a semiconductor material such as silicon or gallium arsenide,or an inorganic oxide such as alumina, titania, or zirconia.

The selection of the solder for joining the integrated circuit deviceand the printed circuit board depends upon several factors. For example,the solder should be compatible with the metal or metals used to formthe leads of the integrated circuit device and the printed circuit board(i.e., upon removal of oxides from said metals by the fluxing agent thesolder wets the leads during reflow to form an electrically conductivebond). Additionally, the selection of the solder may depend uponenvironmental and/or worker safety concerns. For example, there is anever increasing demand for lead-free solders. Still further, the solderalloy preferably melts at a sufficiently low temperature so that thereis no degradation of the integrated circuit device or the printedcircuit board.

Also, the solder preferably melts at a temperature at which thethermoplastic fluxing underfill is stable. For example, in oneembodiment the solder melts at a temperature that is less than about300° C. In another embodiment the solder melts at a temperature betweenabout 180° C. and about 260° C. In yet another embodiment the soldermelts at a temperature between about 220° C. and about 260° C. Further,when performing a reflow operation, the reflow temperature is typicallyabout 10° C. to about 40° C. higher than the solder alloy melttemperature. For example, when reflowing a solder alloy having arelatively high melting temperature, for example, a melting point thatis between about 210° C. and about 240° C., a reflow temperature that isbetween 220° C. about 260° C. is typically preferred. When reflowing asolder alloy having a relatively low melting temperature, for example, amelting point that is between about 160° C. and about 190° C., arelatively low reflow temperature that is between about 170° C. andabout 225° C. is generally preferred.

In view of the foregoing, the thermoplastic fluxing underfill of thepresent invention may be used with any conventional leaded solders(e.g., Sn₆₃Pb₃₇ and Sn₆₂Pb₃₆Ag₂). However, it is particularly usefulwith solder alloys that are substantially free of lead which arecommonly referred to as Pb-free solder alloys and typically contain lessthan about 0.3 wt % of lead. Pb-free solder alloys tend to have higherliquidus temperatures and/or reflow durations than lead-containingsolder alloys. Exemplary Pb-free solder alloys include: Au₈₀Sn₂₀,Sn_(96.2)Ag_(2.5)Cu_(0.8)Sb_(0.5), Sn₆₅Ag₂₅Sb₁₀, Sn_(96.5)Ag_(3.5),Sn_(5.5)Ag_(3.8)Cu_(0.7), Sn_(96.5)Ag₃Cu_(0.5), Sn_(95.5)Ag₄Cu_(0.5),Sn_(93.6)Ag_(4.7)Cu_(1.7), Sn₄₂Bi₅₈, Sn₉₀Bi_(9.5)Cu_(0.5),Sn_(99.3)Cu_(0.7), Sn₉₉Cu₁, Sn₉₇Cu₃, Sn_(87.1)In_(10.5)Ag₂Sb_(0.4),Sn_(77.2)In₂₀Ag_(2.8), Sn_(63.6)In_(8.8)Zn_(27.6), Sn₉₇Sb₃ and Sn₉₅Sb₅.The thermoplastic fluxing underfill of the present invention isparticularly suited for fluxing any of the foregoing Pb-free solderalloys.

The solder alloy is typically applied as a solder paste which is amixture of powdered solder metal alloy suspended or dispersed in aliquid vehicle. In general, at room temperature the solder paste iscompliant enough so that it can be made to conform to virtually anyshape. At the same time, it is “tacky” enough that it tends to adhere toany surface it is placed into contact with. These qualities make solderpaste useful for forming solder bumps on electronic components such asball grid array packages (BGAs) or on the board to attach BGAs.Typically, the solder paste is deposited by stenciling or screenprinting. In one embodiment the solder paste is deposited onto thesolder-wettable pads of the integrated circuit device. In anotherembodiment the solder paste is deposited onto the solder-wettable padsof the printed circuit board. In yet another embodiment solder paste isdeposited on both the solder-wettable pads of the integrated circuitdevice and the printed circuit board.

The selection of the thermoplastic resin is based, in large part, on itsthermal-related properties. For example, the thermoplastic resinpreferably readily flows at reflow temperatures to minimize theoccurrence of voids in the underfill and thereby maximize the bondingbetween the underfill, the integrated circuit device, and the printedcircuit board. More specifically, in one embodiment of the presentinvention the viscosity of the thermoplastic resin at temperatures at orabove the melting point of the solder alloy (e.g., between about 220° C.and about 260° C.) is less than about 30,000 cP. In another embodimentthe viscosity of the thermoplastic resin is between about 10,000 andabout 1,000 cP at a temperature between about 220° C. and about 260° C.In still another embodiment the viscosity of the thermoplastic resin isbetween about 3,000 and about 300 cP at a temperature between about 180°C. and about 240° C.

Additionally, the thermoplastic resin is selected to have sufficienttack to hold the integrated circuit device to the printed circuit boardwhen mounting the integrated circuit device using, for example, a pickand place machine available from, e.g., Assemblion or Siemens.Specifically, in one embodiment the thermoplastic resin has sufficienttack for mounting the integrated circuit device upon being heated tobetween about 80° C. and about 125° C. As such temperatures, theviscosity of the thermoplastic resin is between about 2,500,000 andabout 100,000 cP.

These thermal properties are, in large part, dependent upon the glasstransition temperature (Tg) and the melt temperature (Tm) of thethermoplastic resin. The glass transition temperature is the temperatureat which the polymer transforms from being solid-like and exhibiting anelastic deformation profile to being rubber-like and exhibiting aviscous deformation profile. Additionally, the transformation at Tg istypically associated with a substantial increase in the coefficient ofthermal expansion (CTE). The melt temperature of the polymer is thepoint at which significant dimensional deformation (e.g., between about1 and about 5%) under a load of about 25mN occurs during a statictemperature ramp utilizing a thermomechanical analyzer. It has beendiscovered that thermoplastic resins having sufficient flowability atsolder reflow temperatures, sufficient tack for mounting an integratedcircuit device, and reworkability have a Tg that is between about −25°C. and about 60° C. A thermoplastic resin with a Tg lower than about−25° C. would most likely have too low of a viscosity at the maximumreflow temperature and flow out from an integrated circuit device duringthe reflow operation. In contrast, a thermoplastic resin with a Tg aboveabout 60° C. would tend not to flow sufficiently to form the desiredbond between the integrated circuit device and printed circuit board. Ithas also been discovered that appropriate thermoplastic resins typicallyhave a Tm that is within the range of about 50° C. to about 150° C. Inone embodiment of the present invention the thermoplastic resin has a Tgthat is between about −15° C. about 40° C. and a Tm that is betweenabout 60° C. and about 150° C. In yet another embodiment thethermoplastic resin has a Tg that is between about 20° C. and about 40°C. and a Tm that is between about 80° C. and about 100° C. In stillanother embodiment the thermoplastic resin has a Tg that is betweenabout 25° C. and about 35° C. and a Tm that is between about 85° C. andabout 95° C. In yet another embodiment the Tg is between about −5° C.and about 10 EC and the Tm is between about 50° C. and about 65° C.

In addition to temperature, the viscosity of the thermoplastic resin isrelated to the molecular weight of the polymer. In general, as themolecular weight of the polymer increases or decreases, so does theviscosity of the thermoplastic resin at a particular temperature. In oneembodiment the molecular weight of the thermoplastic resin is betweenabout 30,000 and about 55,000 daltons. In other embodiment the molecularweight of the thermoplastic resin is between about 30,000 and about40,000 daltons. In yet another embodiment the molecular weight of thethermoplastic resin is between about 30,000 and about 36,000 daltons. Instill another embodiment the molecular weight is between about 34,000and about 42,000 daltons. In another embodiment the molecular weight ofthe thermoplastic resin is between about 42,000 and about 55,000daltons.

The selection of the thermoplastic resin is also based on its thermalstability (i.e., its resistance to degradation at elevatedtemperatures). Stated another way, the selected thermoplastic resin isconsidered to be thermally stable (i.e., it does not substantiallydegrade during a reflow operation or a subsequent release/reworkoperation). The thermal stability of a thermoplastic resin may bequantified in terms of weight loss when heated to a particulartemperature for a particular duration. With respect the presentinvention, a thermoplastic resin is considered to be thermally stable ifthe weight loss is less than about 10 percent when subjected to athermogravimetric analysis comprising heating the resin to at least themaximum temperature at which the desired reflow operation occurs for atleast the duration at which the maximum temperature is maintained. Forexample, in one embodiment the thermoplastic resin has less than about 5percent weight loss when heated to about 250° C. for about 60 secondsand is considered to be thermally stable. In another embodiment thethermoplastic resin has less than a 10 percent weight loss when heatedto about 300° C. for about 60 seconds and is considered to be thermallystable. Additionally, the thermoplastic resin is also preferablymoisture resistant.

It has been discovered that thermoplastic resins having propertiessuitable for such a demanding application are phenoxy-based polymers ofbisphenol A (i.e., they comprise polyhydroxyether). Other appropriatethermoplastic resins include polysulfone. One such commerciallyavailable phenoxy-based resin is INCHEMREZ PHENOXY PKCP-80 availablefrom the InChem. Corporation. This resin is a phenoxy resin having about20 weight percent of caprolactone grafted onto the backbone hydroxylgroups. The INCHEMREZ PHENOXY PKCP-80 has a molecular weight of about39,000 daltons and a glass transition temperature of about 30 EC bydifferential scanning calorimetry. The caprolactone decreases theviscosity of the thermoplastic resin. Additionally, the caprolactonetends to decrease the Tg of the phenoxy resin which without out thecaprolactone would be about 90° C.

The PKCP-80 resin adequately flows at reflow temperatures. Specifically,the viscosity of the PKCP-80 resin is between about 7,000 and about2,500 cP at a temperature between about 220° C. and about 260° C. Also,the PKCP-80 resin has sufficient tack for mounting an integrated circuitdevice. Specifically, the viscosity of the resin is between about100,000 and about 500,000 cP at a temperature between about 80° C. andabout 125° C. The PKCP-80 resin is also considered to be thermallystable. Specifically, upon being heated to a temperature of about 250°C. for about 90 seconds, the resin only loses about 2% of its weight.Further, upon being heated to a temperature of about 300° C. for about90 seconds, the resin loses less than about 5% of its weight. With suchthermal stability, the material is considered to not decompose during asolder reflow operation. The PKCP-80 also exhibits a low moistureuptake, specifically, less than about 5% when heated to about 130° C.while exposed to a 85% relative humidity atmosphere.

By utilizing a thermoplastic resin system (i.e., a polymer that softenswhen exposed to heat and returns to its original condition when cooledto room temperature), the underfill of the present invention isreworkable following the reflow operation. Thermoplastic resinstypically comprise very little cross-linking of the polymer moleculeswhich allows greater molecular mobility and hence the ability to softenwhen heated. In contrast, many conventional underfills comprise“thermoset” resins which are typically highly cross-linked polymers thatcannot be softened after reflow which prevents removal or reworking of afaulty chip.

To form a layer of thermoplastic fluxing underfill, the thermoplasticresin is typically dissolved in an appropriate solvent or solvent blend.The particular solvent or solvents is not overly critical, but thesolvent should readily dissolve the thermoplastic resin and becompatible with the components in the thermoplastic fluxing underfill.Preferably, the solvent also has evaporation and boiling points that arehigh enough so that it is considered easy and safe to handle yet lowenough to allow removal of the solvent at room temperature or a dryingoven (e.g., the evaporation point is preferably between about 70° C. andabout 170° C. and the boiling point is preferably between about 90° C.and about 130° C.). Appropriate solvents include many polar solventssuch as ketones (e.g., acetone, methyl ethyl ketone, methyl isobutylketone, cyclohexanone), esters (e.g., ethyl lactate, dibasic esters,ethylene glycol ethylether acetate, diethyleneglycol ethylether acetate,propyleneglycol methylether acetate, hexanediol diacrylate, phenoxyethyl acrylate, ethoxyethyl propionate), alcohols (e.g., methanol,ethanol, isopropyl alcohol, benzyl alcohol, methylcellosolve,ethylcellosolve, 1-methoxy-2-propanol, carbitol and butylcarbitol), andcombinations thereof. In one embodiment of the present invention thesolvent is ethyl-ethoxypropionate and is commercially available fromEastman Chemical of Kingsport, Tenn.

The amount, or concentration, of thermoplastic resin dissolved in thesolvent depends primarily upon the manner in which the thermoplasticfluxing underfill is to be applied to the integrated circuit deviceand/or printed circuit board. In general, the concentration of resin insolution is between about 20 and about 80 percent by weight of thethermoplastic fluxing underfill solution prior to application. However,the concentration may be outside the foregoing range and still be withinthe scope of the present invention. Depending upon the applicationmethod, the concentration of resin will tend to be toward the one end orthe other of the range. For example, if the thermoplastic fluxingunderfill is being deposited as a flowable liquid (e.g., it is beingdispensed by needle and syringe) the concentration of resin is typicallylower (e.g., between about 30 and about 45 weight percent of thethermoplastic fluxing underfill solution). Whereas, if the thermoplasticfluxing underfill is being cast into a film prior to being applied tothe integrated circuit device, the concentration of resin in thesolution tends to be higher (e.g., between about 40 and about 80 weightpercent of the thermoplastic underfill solution). In one embodiment, thesolution comprises about 40 weight percent of INCHEMREZ PHENOXY PKCP-80dissolved in ethyl-ethoxypropionate and the solution is cast into afilm.

The thermoplastic fluxing underfill of the present invention alsocomprises a fluxing component to remove oxides from all surfacesinvolved in the soldering operation (e.g., solder pads, solder bumps,and solder alloy powder). Further, the fluxing component also protectsagainst oxidation during, and for a sufficient duration after, thereflow operation. Additionally, the flux and/or its residues preferablydo not corrode the solder metal prior to, during, or following thesoldering operation.

In addition, the fluxing component is preferably soluble or dispersablein the solvent and thermally stable at reflow temperatures. In general,the flux component comprises a carboxylic acid (e.g., mono-, di- andpolycarboxylic acids). Carboxylic acids and dicarboxylic acids arepreferred fluxing agents for many solder applications, however, manylower molecular weight acids decompose or evaporate at reflowtemperatures. As such, the fluxing component preferably comprises highermolecular weight carboxylic and dicarboxylic acids. For example,carboxylic acids greater than C20 such as behenic acid, abietic acid,urocanic acid and dicarboxylic acids greater than C12 such asdodecanedioic acid and dodecanedicarboxyllic acid are preferred.Although they may be used, many of these materials are solid at roomtemperature and are not very soluble in polar solvents. Preferably, thefluxing agent is a liquid carboxylic acid such as isostearic acid,and/or DIACID 1550 from Westvaco. In one embodiment of the presentinvention the fluxing component comprises a liquid dicarboxylic acidsold under the trade name DIACID 1550 by Westvaco Chemicals ofCharleston, S.C. The DIACID 1550 tends to be soluble in the appropriatesolvents and has an appropriate thermal stability.

To form a completely fused and strong solder joint, the solder mustadequately wet the solder pad and/or lead. Wetting depends in large parton the metallurgical reaction between solder and soldering surface, andon the efficacy of any fluxing component. Thus, if the fluxing componentdoes not adequately remove oxides from the metals being joined duringthe reflow operation, the oxides retard or prohibit the reaction.Additionally, the joint will typically be incompletely fused, weak, andsubject to forming a void in the solder joint. Without being held to aparticular theory, it is presently believed that the mechanism behindvoid formation is the entrapment of excess flux or its vapors within thesolder alloy. Thus, in addition to being thermally stable, theconcentration of fluxing component in the thermoplastic fluxingunderfill solution should be sufficient to reduce the metal oxides inthe solder alloy and on the solderable surfaces, but not so great as tocreate voids. Typically, this is accomplished with a concentration offluxing component that is between about 1 and about 10 weight percent ofthe thermoplastic fluxing underfill solution. In another embodiment theconcentration of fluxing component is between about 4 and about 7 weightpercent of the thermoplastic fluxing underfill solution. In yet anotherembodiment the concentration is about 4 weight percent of thethermoplastic fluxing underfill solution. In still another embodimentthe concentration is about 2.5 weight percent of the thermoplasticfluxing underfill solution.

Although not required, other additives, such as wetting agents,defoaming agents, and coefficient of thermal expansion (CTE) modifiersmay be added to the thermoplastic fluxing underfill. A wetting agent istypically added to improve the film forming properties of the underfilland/or to enhance the bonding of the underfill to the surfaces of theintegrated circuit device and printed circuit board by decreasing thesurface tension of the underfill. Appropriate wetting agents include thefollowing classes of materials: modified silicone resins, fluorocarbons,and acrylic resins. The most commonly used type of wetting agent inunderfills are silanes. In one embodiment the thermoplastic fluxingunderfill comprises a commercially available silane-type wetting agentfrom Byk Chemie of Wesel, Germany sold under the trade name BYK 306. TheBYK 306 wetting agent only contains 12 percent by weight wetting agentwith the remainder being solvent. If present, the concentration of awetting agent in the thermoplastic fluxing is typically kept near theminimum concentration at which effective wetting is accomplished becausehigh concentrations can actually decrease adhesion. In general, theconcentration of wetting agent in the underfill is between about 0.005and about 2.0 weight percent of the solution. In one embodiment theconcentration of wetting agent is between about 0.05 and about 0.20weight percent of the thermoplastic fluxing underfill solution. In oneembodiment the thermoplastic fluxing underfill comprises about 1 weightpercent of BYK 306 which in terms of what is actually added to thethermoplastic fluxing underfill solution is about 0.12 weight percent ofthe wetting agent and about 0.88 weight percent of the associatedsolvent.

Defoaming agents are typically added prior to, or during, the mixing ofthe thermoplastic resin and solvent to assist in the degassing of theunderfill solution. Stated another way, a defoaming agent tends tominimize the formation of pockets of entrapped air in the underfillsolution. Such pockets of entrapped air tend to result in the formationof voids in the cured underfill which can degrade the adhesion andthermal stress compensation of the underfill. Appropriate defoamingagents include the classes of materials of polyether modified siloxanesand methylalkyl siloxanes. The most commonly used type of defoamingagent in underfills are modified polysiloxanes. Specific examples ofunderfill defoaming agents include BYK 525, BYK 530, and BYK 535available from Byk Chemie of Wesel, Germany. In one embodiment thethermoplastic fluxing underfill comprises a commercially availablemodified polydimethylsiloxane-type defoaming agent from Crompton ofMiddlebury, Conn. sold under the trade name SAG 100. If present, theconcentration of a defoaming agent in the thermoplastic fluxing istypically kept near the minimum concentration at which effectivedegassing is accomplished because high concentrations can decreaseadhesion. In general, the concentration of defoaming agent is no greaterthan about 1 weight percent of the thermoplastic fluxing underfillsolution. For example, in one embodiment the thermoplastic fluxingunderfill comprises about 1 weight percent of SAG 100. In anotherembodiment the concentration of defoaming agent is between about 0.05and about 0.5 weight percent of the solution. In yet another embodimentthe concentration of defoaming agent is about 0.10 weight percent of theunderfill solution.

A thermoplastic resin as set forth above typically has a coefficient ofthermal expansion (CTE) that is between about 20 and about 70 ppm/EC andacts to reduce the CTE mismatch between the solder and the substratematerials. To further reduce any CTE mismatch between the integratedcircuit, the solder, and the circuit board, the thermoplastic fluxingunderfill of the present invention may optionally comprise a coefficientof thermal expansion modifier component. The CTE modifying component hasa CTE that is more compatible with the substrates (e.g., the flip chipand circuit board) thereby decreasing the thermal stress upon thermalcycling. The CTE modifying component is electrically insulating and hasa CTE that is preferably less than about 10 ppm/° C. Exemplary CTEmodifying component materials include beryllium oxide (about 8.8 ppm/°C.), aluminum oxide (about 6.5-7.0 ppm/° C.), aluminum nitride (about4.2 ppm/° C.), silicon carbide (about 4.0 ppm/° C.), silicon dioxide(about 0.5 ppm/° C.), low expansion ceramic or glass powders (betweenabout 1.0 to about 9.0 ppm/° C.), and mixtures thereof. In oneembodiment of the present invention the CTE modifying componentcomprises silicon dioxide.

The maximum particle size of the CTE modifying component (i.e., themaximum cross-sectional distance of the particle) is preferably lessthan the height of the solder bumps to minimize any negative impact onsolder joint integrity. Typically, the average particle size of the CTEmodifying component is between about 3 and about 15 μm. Although theamount of the CTE modifying component in the thermoplastic fluxingunderfill depends on the particular application, if present, the CTEmodifying component typically comprises between about 10 and about 90 wt% of the thermoplastic fluxing underfill.

In general, the thermoplastic fluxing underfill solution is prepared bymixing together the various constituents. Typically, the preparationprocess comprises heating the solvent and thermoplastic resin to enhancethe rate of dissolution. After dissolution of the resin in the solventis complete, any remaining constituents such as wetting agents,defoaming agents, and CTE modifiers are typically dissolved or mixedinto the solution.

As set forth above, the underfill solution may be formulated to have thecorrect rheology for the method of application. For example, because theratio of solvent to solids is the primary factor in determining theviscosity of the solution, it is possible to formulate underfillsolutions that can be applied using different methods. Additionally,because the solvent is substantially entirely evaporated afterapplication of the underfill solution to the integrated circuit devicewafer, the resulting, solid underfill layer will have the samecomposition regardless of the initial viscosity and percent solids ofthe underfill solution. This is because the solvent is merely a vehiclefor carrying the solids during underfill application.

In one application method, the underfill solution is applied by spincoating. Spin coating is a common semiconductor processing method inwhich liquid is deposited onto a spinning wafer in order to provide asmooth and level coating. A typical viscosity for spin coating anunderfill is between about 80 and about 85 Kcps, measured at 2.5 RPMusing an RVT #6 spindle on a Brookfield viscometer. When applied to awafer, a wafer spin rate of between about 700 and about 1500 RPM hasbeen found to yield to uniform and smooth coating. Good applicationresults have been found with a wafer spin rate of about 1200 RPM.

A second method for applying the thermoplastic fluxing underfill isstencil printing. This method typically requires a more viscous solutionthan that for spin coating. The thixotropic index, (i.e., change inviscosity as a result of mechanical shearing), can also be adjusted,using thixotropic additives, to improve printing characteristics.Specifically, the rheology of the solution is preferably gel-like orsemi-solid if static, however, when a shear force is applied itpreferably flows like a liquid. This allows for the underfill solutionto flow through a stencil when a force is applied using a squeegee andto maintain the pattern of the stencil after the stencil is removed fromthe surface of the substrate. Exemplary thixotropic agents include fumedsilicas. If present, the thixotropic agents typically comprise betweenabout 0.2 and about 9 weight percent of the underfill solution. Thethickness of the stencil determines the amount of material applied tothe wafer and the stencil should be thicker than the bump height so thatthe blade applying the underfill material does not contact the bumps. Ifsuch contact does occur, damage to the bumps or even displacement of thebumps may occur.

The print method employs the use of a metal stencil and an automatedstencil print machine such as those available from Speedline. In thismethod, the liquid underfill is deposited on the metal stencil which hasan aperture slightly larger than the array, and a squeegee, either metalor rubber, is used to wipe the material over the aperture. The device tobe underfilled is fitted in a tray (i.e., a JEDEC tray) or holdingdevice with the array exposed in the stencil aperture. The material isdeposited on the device via the wiping of the material over theaperture. The process parameters such as aperture height, collapseheight, and percent solids of the thermoplastic composition aretypically fine tuned to result in void free joint formation.

Other well known methods of depositing liquid underfill onto anintegrated circuit device, wafer, or other substrate include spraying,screen printing, and needle deposition. Regardless of which manner theliquid underfill is applied, after being applied at least a portion ofthe solvent is evaporated from the underfill solution thereby increasingthe viscosity of the underfill. Typically, the evaporation is enhancedby heating the underfill solution in an oven or by direct heating of thewafer. It has been found to be advantageous to heat the wafer whilesimultaneously using a forced hot air oven to help drive solvent out ofthe coating. Combined top and bottom heating can eliminate any tendencyto trap solvent in the underfill layer by a process known as “skinning”in which the surface of the underfill material dries prematurely andforms a film (i.e., a skin) that acts as a barrier to further solventevacuation. If drying is carried out properly, the resulting underfillmaterial is non-tacky and amenable to handling. If a slight degree oftackiness at room temperature is desired, however, a tackifier may beadded to the underfill.

It is generally preferred that the thickness of the dried underfillmaterial be less than the height of the solder bumps to allow forcollapse of the bumps during the reflow operation. In one embodiment,the thickness of the dried underfill layer is between about 50 and about80 percent of the solder bump height. In another embodiment thethickness of the dried underfill layer is between about 60 and about 70percent of the solder bump height. The amount of solvent contained inthe underfill solution determines the amount of thickness reduction thatoccurs in the underfill during drying and solvent evacuation. Thus, inaddition to stencil thickness, for example, the amount of solvent in theunderfill solution and/or the deposit thickness may be controlled inorder to control the thickness of the applied underfill. Typically, adry underfill thickness range of about 25 to about 125 microns issuitable depending on the height of the solder bumps.

Alternatively, the underfill may be applied to the integrated circuitdevice as a solid underfill layer. Specifically, the underfill solutionmay be cast onto a release substrate (e.g., paper) and then dried into afilm. The resulting film is then typically cut into a proper shapecalled a preform and applied to the integrated circuit device wafer(i.e., a wafer comprising a multiplicity of integrated circuit devices).Heating, with the application of pressure or a vacuum, is typically usedto bond the underfill layer to the wafer. The temperature of the layerpreferably is not increased above the point at which the fluxingproperties of the underfill are activated (e.g., the temperature may beabout 175° C.). Pulling a vacuum is generally preferred over applyingpressure because it tends to be more effective at preventing air frombeing trapped between the film and the chip. One advantage of a solidfilm is that it can be easily shipped, conveniently stored, and appliedby simple mechanical handling equipment. Like the underfill layersapplied as a liquid to the integrated circuit, the film thickness shouldbe less than the height of the solder bumps. In fact, the foregoing drythickness range is equally applicable.

Unlike systems which employ a separate flux and underfill, the presentsystem allows the underfill material to cover the solder bumps since itoffers fluxing properties as well as underfill properties. In fact, itis preferred that the material cover the bumps because, in so doing, thebumps will be protected from oxidation, contamination, and mechanicaldamage. Each of the application methods described above has thecapability of covering the bumps with the underfill material.

At this stage, the wafer is ready to be diced, or singulated, to produceindividual area array devices (e.g., flip chips). Any of a wide varietyof the methods known in the art for dicing wafers can be employed tothat end. The sole requirement is that the process does not degrade theunderfill material applied to the wafer/chip surface(s). In oneembodiment dicing is achieved by attaching the wafer to a holding tapeand then sectioning the wafer using, for example, a DISCO saw with a 5μm diamond cutting blade operating at a speed of about 30,000 rpm. Waterjet cooling is used to keep the temperature at the cut below thesoftening point of the film. The individual die or chip can then bepicked off the tape and placed into waffle packs, tape and reelpackaging, or other convenient die presentation systems used in theindustry.

Once diced, individual area array devices may now be bonded to circuitboards and the like. Each area array device is placed and aligned to thebond pads of a substrate. As used herein, the term “substrate” isintended to mean a circuit board, a chip carrier, another semiconductordevice, or a metal lead frame. It is not necessary to add flux, althoughflux may be added for special reasons such as compensating for excessiveoxide on substrate pads, or the need to hold the flip chip in placeduring assembly (if the underfill is not tacky at room temperature orheated until tacky).

The area array device is then placed on the substrate using a pick andplace machine. If the underfill is not tacky at room temperature, thesubstrate is preferably heated to a temperature within the range ofabout 80° C. to about 120° C. so that the thermoplastic has tack enoughto hold the die in place. The positioned chip and substrate assembly isthen typically run through a multi-zone oven with individual heatcontrols that permit a heating profile appropriate for the specificsolder. During reflow, the flux in the underfill reduces oxides presenton the solder or the metal surface in contact with the solder and allowssolder joints to form at the substrate and circuit device pads. Further,within the temperature range of about 60° C. to about 130° C. thethermoplastic resin softens sufficiently to flow and wet the integratedcircuit device and substrate surfaces. The assembly is cooled and thesolder and underfill harden to form a bonded assembly comprising theintegrated circuit device, the substrate, at least one solder joint, andthe underfill.

Alternatively, a flip chip bonder that can apply heat and pressure maybe employed instead of the reflow oven. In this embodiment, theintegrated circuit assembly (flip chip coated with the thermoplasticfluxing underfill) is placed in contact with the conductive pads on thecircuit board and heat from the bonder head softens the underfillthereby activating the flux, reflowing the solder bumps, and softens theunderfill to bond to the board and chip. The use of a flip chip bonderallows a flip chip to be assembled to a board that already hascomponents mounted thereto. This method may also be used to attach achip to a site that is being reworked.

Reworking is desirable, for example, if a chip mounting step has failedto properly position the chip on the board. Specifically, the assemblyof fine pitch, high-density components can result in misalignments andfailed connections. Furthermore, because it is difficult to fully testan unpackaged device such as a flip chip, it is desirable to be able toremove the chip if final testing indicates that the chip is notoperating optimally, either through a fault with the chip or as a resultof improper mounting. Thermoset underfills do not allow the assembly tobe reworked since thermosets cannot be melted once they havecrosslinked. The present invention eliminates the problems associatedwith thermoset underfills by incorporating a thermoplastic resin as themain component of the underfill. Thus, a previously bonded chip may beremoved by raising the chip temperature to above the melting point ofthe solder (approximately 183 EC for tin/lead solder) and above thede-bonding temperature of the underfill resin. Typically, the reworktemperature is about 15 to about 25° C. above the solder reflowtemperature. Although, the temperature may be higher if localized heat,such as produced with a chip bonder, is used.

The invention can be further understood with reference to FIGS. 1 and 2.As can be seen schematically in FIG. 1, a semiconductor device 10comprises a portion of a semiconductor wafer 12 having solder bumps 14applied to its surface. Subsequently, as represented schematically inFIG. 2, the device 10 has had a flux/underfill material 16 applied tothe surface of the wafer 12 having the solder bumps 14. The underfillmaterial 16 occupies at least the spaces between the bumps 14 and alsocovers the bumps.

With the foregoing method and compositions, a thermoplastic fluxingunderfill and an assembly comprising the underfill may be produced. Thethermoplastic fluxing underfill of the present invention providesseveral advantages such as: extended shelf life stability (e.g., greaterthan six months); provides mechanical shock resistance; delays orprevents device failure do to thermal cycling; reworkability;application to chips eliminates the need to underfill at the end userfacility; decreased manufacturing costs; and may be used withlead-containing and lead-free solder.

The following Examples will help to illustrate the invention further.

EXAMPLES

A thermoplastic fluxing underfill containing about 40.00 wt % phenoxyPKCP-80 thermoplastic resin, about 55.90 wt % of ethyl3-ethoxypropionate solvent, about 1.00 wt % of BYK 306 wetting agent,about 0.10 wt % of SAG 100 defoaming agent, and about 4 wt % DIACID 1550dicarboxylic acid fluxing agent was prepared according to the followingsteps. The solvent was placed in a stainless steel beaker and heated toa temperature of about 70° C. while being stirred. Then thethermoplastic resin was added to the solvent in approximately tenpercent portions. Specifically, each 10 percent portion was added to thesolvent while the stirring was maintained until completely dissolved. Atthat point, the next ten percent portion was added and mixed. This wasrepeated until all of the thermoplastic resin was dissolved in thesolvent at which time the heating of the solution was discontinued. TheBYK 306 and SAG 100 were then added. The solution was allowed to cool toroom temperature and the solids content was about 58 percent and theviscosity was about 9,000 cP.

The foregoing thermoplastic underfill solution was cast onto a papersuch that the wet film had a thickness of about 0.5 mm. The cast wetfilm was then dried at a temperature of about 120° C. to evaporate thesolvent. The dried film had a thickness of about 0.25 mm. A section ofthe dried film was cut and placed on the solder bumps of a ball gridarray. The solder balls comprised a eutectic tin-lead alloy. The twowere placed in an oven maintained at about 165° C. to attach theunderfill layer to the flip chip. The underfill coated flip chip wasthen placed on a printed circuit board and subjected to a reflowoperation having a maximum temperature of about 225° C. and a durationof about 90 seconds.

Integrated circuit assemblies comprising the integrated circuit deviceand the thermoplastic fluxing underfill were then evaluated forresistance to mechanical shock and thermomechanical stress failures.Specifically, the assemblies were tested using a drop shock test whichis used to determine resistance to mechanical shock caused by dropping aproduct containing the assembly. The thermoplastic fluxing underfillimproved the drop shock performance of the area array devices by afactor of at least 10 over devices with no underfill. Drop shockperformance is a key indicator of the robustness of the deviceconnection to the board. The process involves attaching a 50 g weight toa circuit board with 10 devices and then allowing the weighted board tofall about 2 meters before striking a horizontal surface. A failure isrecorded when a device detaches from the board or when the electricalcontinuity for the device goes to open. Assemblies containing thethermoplastic fluxing underfill were also subjected to a thermal shockreliability test to determine the resistance to stress caused by thermalcycling between −40° C. and 125° C. The thermoplastic fluxing underfillpassed this test by undergoing 1000 cycles with less than a 50 percentfailure rate.

A second thermoplastic fluxing underfill solution was prepared with alow molecular weight diluent thermoplastic resin to reduce the overallTg and Tm of the thermoplastic resin component of the underfill toevaluate the applicability of the underfill for devices which requirerelatively low viscosity for collapse at typical or relatively lowreflow temperatures. The formulation comprised about 28 weight percentof PKCP-80, about 8 weight percent of ethoxylated bisphenol Athermoplastic resin (Aldrich Chemical), about 62 weight percent ofcyclohexanone, about 2.5 weight percent of DIACID 1550, and about 1weight percent BYK 306. The constituents were placed in an eight ouncepolypropylene jar that was rolled at about 100 rpm for about 2 days on aball mill roller. Because this manufacturing process is a closed systemand uses no heat, the process reduces or eliminates the evaporation ofsolvent during the incorporation process. Advantageously, becauseevaporation is greatly reduced or eliminated, this method allows for thepreparation of an underfill solution in a consistent manner. Thematerial was then dispensed via syringe onto eutectic bumped 10 mm×10 mmAmkor CABGA (daisy chained) devices. The devices with the coatingsolution were dried for 1 hour at 70° C. then for 1 hour at 165° C. Acoating of approximately 85% of the solder ball height was achievedafter the deposition and drying process was completed twice.

The coated 10 mm×10 mm devices were then dipped in a commercial flux,placed via hand on an FR4 substrate, and passed through a reflow profileto form interconnect and to melt the pre applied underfill. Drop shockdata indicated a significant improvement in device survivability(defined as an electrical open in the daisy chain) to approximately 10times that of devices without an underfill.

It is to be understood that the above description is intended to beillustrative and not restrictive. Many embodiments will be apparent tothose of skill in the art upon reading the above description. The scopeof the invention should therefore be determined not with reference tothe above description alone, but should be determined with reference tothe claims and the full scope of equivalents to which such claims areentitled.

1. A method for forming an integrated circuit assembly for attachment toa circuit board by soldering, the method comprising: applying anunderfill solution comprising a thermoplastic resin, a solvent, and afluxing agent to an integrated circuit device having at least one solderbump on a surface thereof such that the underfill solution is in contactwith the at least one solder bump and with the surface of the integratedcircuit device; and removing at least a portion of the solvent from theapplied underfill solution to thereby yield the integrated circuitassembly for attachment to a circuit board, wherein the integratedcircuit assembly comprises the integrated circuit device, the at leastone solder bump, and a thermoplastic fluxing underfill in contact withthe integrated circuit device surface and in contact with the at leastone solder bump.
 2. The method of claim 1 wherein removing at least aportion of the solvent from the applied under fill solution yields acured thermoplastic fluxing underfill in contact with the integratedcircuit device surface.
 3. The method of claim 1 wherein thethermoplastic resin has a viscosity between about 2,500,000 cP and about100,000 cP at a temperature between about 80° C. and about 125° C. 4.The method of claim 1 wherein the thermoplastic resin has a viscositybetween about 100,000 cP and about 500,000 cP at a temperature betweenabout 80° C. and about 125° C.
 5. The method of claim 1 wherein thethermoplastic resin has a viscosity of less than about 30,000 cP at atemperature between about 220° C. and about 260° C.
 6. The method ofclaim 1 wherein the thermoplastic resin has a glass transitiontemperature that is between about −25° C. and about 60° C., a molecularweight that is between about 30,000 and about 50,000 daltons, and aviscosity that is between about 10,000 and about 1,000 cP at atemperature that is between about 220° C. and about 260° C.
 7. Themethod of claim 6 wherein the thermoplastic resin is a phenoxy resinhaving about 20 weight percent of caprolactone grafted onto the backbonehydroxyl groups, the solvent is a polar solvent selected from the groupconsisting of a ketone, an ester, and an alcohol, and the fluxing agentis selected from the group consisting of a monocarboxylic acid havingmore than 20 carbon atoms per molecule, and a dicarboxylic acid havingmore than 12 carbon atoms per molecule that are liquid at roomtemperature and are soluble in the solvent.
 8. The method of claim 7wherein the underfill solution comprises a concentration of thethermoplastic resin that is between about 20 and about 60 weightpercent, a concentration of the solvent that is between about 40 andabout 80 weight percent, and a concentration of the fluxing agent thatis between about 1 and about 10 weight percent.
 9. The method of claim 2wherein the thermoplastic resin has a viscosity between about 2,500,000cP and about 100,000 cP at a temperature between about 80° C. and about125° C.
 10. The method of claim 2 wherein the thermoplastic resin has aviscosity between about 100,000 cP and about 500,000 cP at a temperaturebetween about 80° C. and about 125° C.
 11. The method of claim 2 whereinthe thermoplastic resin has a viscosity of less than about 30,000 cP ata temperature between about 220° C. and about 260° C.
 12. The method ofclaim 2 wherein the thermoplastic resin has a glass transitiontemperature that is between about −25° C. and about 60° C., a molecularweight that is between about 30,000 and about 50,000 daltons, and aviscosity that is between about 10,000 and about 1,000 cP at atemperature that is between about 220° C. and about 260° C.
 13. Themethod of claim 12 wherein the thermoplastic resin is a phenoxy resinhaving about 20 weight percent of caprolactone grafted onto the backbonehydroxyl groups, the solvent is a polar solvent selected from the groupconsisting of a ketone, an ester, and an alcohol, and the fluxing agentis selected from the group consisting of a monocarboxylic acid havingmore than 20 carbon atoms per molecule, and a dicarboxylic acid havingmore than 12 carbon atoms per molecule that are liquid at roomtemperature and are soluble in the solvent.
 14. The method of claim 13wherein the underfill solution comprises a concentration of thethermoplastic resin that is between about 20 and about 60 weightpercent, a concentration of the solvent that is between about 40 andabout 80 weight percent, and a concentration of the fluxing agent thatis between about 1 and about 10 weight percent.
 15. The method of claim1 wherein removing at least a portion of the solvent from the appliedunderfill solution dries the applied underfill solution and the processfurther comprises the steps of: placing the integrated circuit assemblyonto the circuit board to yield a circuit board with the integratedcircuit assembly placed thereon; and heating the circuit board with theintegrated circuit assembly placed thereon to a reflow temperature tothereby solder the integrated circuit device to the circuit board whilethe fluxing agent fluxes the solder and to flow the cured thermoplasticfluxing underfill thereby yielding a circuit board having the integratedcircuit device attached thereto with a metallic solder connection andthe cured thermoplastic underfill between and bonded to the circuitboard and the integrated circuit device.
 16. The method of claim 2wherein removing at least a portion of the solvent from the appliedunderfill solution dries the applied underfill solution and the processfurther comprises the steps of: placing the integrated circuit assemblyonto the circuit board to yield a circuit board with the integratedcircuit assembly placed thereon; and heating the circuit board with theintegrated circuit assembly placed thereon to a reflow temperature tothereby solder the integrated circuit device to the circuit board whilethe fluxing agent fluxes the solder and to flow the cured thermoplasticfluxing underfill thereby yielding a circuit board having the integratedcircuit device attached thereto with a metallic solder connection andthe cured thermoplastic underfill between and bonded to the circuitboard and the integrated circuit device.
 17. A method for forming anintegrated circuit assembly for attachment to a circuit board bysoldering, the method comprising: applying a cured underfill filmcomprising a thermoplastic resin and a fluxing agent to an integratedcircuit device having at least one solder bump on a surface thereof suchthat the cured underfill film is in contact with the at least one solderbump and with the surface of the integrated circuit device; and adheringthe cured underfill film to the integrated circuit device to yield theintegrated circuit assembly for attachment to a circuit board, whereinthe integrated circuit assembly comprises the integrated circuit device,the at least one solder bump, and the cured thermoplastic fluxingunderfill adhered to the integrated circuit device surface and the atleast one solder bump.
 18. The method of claim 17 wherein thethermoplastic resin has a viscosity between about 2,500,000 cP and about100,000 cP at a temperature between about 80° C. and about 125° C. 19.The method of claim 17 wherein the thermoplastic resin has a viscosityof less than about 30,000 cP at a temperature between about 220° C. andabout 260° C.
 20. The method of claim 17 wherein the thermoplastic resinhas a glass transition temperature that is between about −25° C. andabout 60° C., a molecular weight that is between about 30,000 and about50,000 daltons, and a viscosity that is between about 10,000 and about1,000 cP at a temperature that is between about 220° C. and about 260°C., and the fluxing agent is selected from the group consisting of amonocarboxylic acid having more than 20 carbon atoms per molecule, and adicarboxylic acid having more than 12 carbon atoms per molecule.